Summary. Intracellular recordings have been performed from interneurons in the lumbar segments of the spinal cord of non‐anesthetized spinal cats. A full account is given of the experimental technique used in this and other investigations in the same series. Section I. Continuous inkwriter recordings of the membrane potential level obtained from interneurons in various parts of the cord cross section are presented and compared with records from afferent fibres and motoneurons. Data of the average amplitude values and recording times for the different types of units are given. Various factors influencing the values of the recorded membrane potential are discussed and an attempt has been made to find a relation hetween the degree of injury, cell size and recorded membrane potential. Section II. The most common types of interneuron discharges in response to synchronous afferent volleys are described and typical variations in prepotential configuration are illustrated. The firing level at which an impulse is set up by afferent single shock stiniulation has been analysed in some detail in an experiment in which physiological variations in the membrane potential level were induced by natural sensory stimulation. It was found that the firing level is not constant but is reduced with lowering of the membrane potential value in an approximately linear way. The spike repolarization level was also found to vary with the nienibrane potential level in a similar way which may result in levels of increased or decreased polarization as compared with the pre‐spike level. Records of spontaneous and naturally induced activity are presented showing the relation between slow depolarization processes and spike discharges. In experiments including both natural and artificial activation records have been obtained showing no distinct prepotentials. In such recordings the spikes have usually been of short duration (average 0.8 msec as compared with 1.3 msec for spikes accompanied by slow potentials). Some of these cases may represent axon recordings, but possible explanations of an absence of recordable prepotentials from soma are also discussed. In the majority of interneuron recordings the spike amplitude has been 10–20 per cent lower than the membrane potential value; overshooting has only been observed in about 10 per cent of the recordings. Some factors influencing the spike amplitude have been taken up for discussion. Special attention has been given to variations in spike configuration in the form of pronounced notches and a splitting up into sub‐spikes; these variations have been interpreted as signs of a fractionated function of the cell membrane.

The ionic efflux during a single action potential of the alga Nitellopsis obtusa was recorded by measuring the increase in conductivity of the extracellular fluid. The conductivity changes are reflected as variations in absorption of a high‐frequency field applied by a specially designed probe placed in the external medium close to the alga and connected to a high‐frequency reflectometer. The combined characteristics of probe/solution and reflectometer are given. Most experiments were performed on algae in distilled water, comparative experiments with lake water having shown that this arrangement does not introduce any significant changes in the activation processes studied. The possibility of dynamic recording of ionic fluxes is illustrated by oscillograph records showing the temporal relation between the ionic concentration change and the action potential; that the method is suitable for quantitative measurements is demonstrated by inkwriter records showing the stepwise increases in concentration caused by intermittent stimulation during hour‐long experiments. Quantitative data on the increments in ionic concentration associated with a single impulse, and on the total increase resulting from a number of stimuli, are presented and compared with chemical analyses showing the ions involved to be K+ and C1‐‐. Satisfactory correspondence was obtained between the values of the conductivity measurements, tentatively expressed as KC1 concentration, and the chemical analysis. The average values for the K+ efflux were 2 times 103 to 4 times 103 pmole/cm2. impulse; preliminary studies showed the C1– efflux to be of approximately the same order.

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The paper presents a method of measuring small variations in electrolytic conductance applicable also to dynamic measurements. By means of a probe immersed in the solution a high‐frequency field is applied, variations in the absorption of which are detected by a special instrument, the H‐F reflectometer, and indicated on separate meters as changes in resistive and capacitive components of the combined impedance, of probe and electrolyte. At the frequency used, 1085 kc/s, absorption maximum occurs at conductivities around 10–4 ohm‐1 cm‐1, at which a change in conductivity of 10‐1 ohm‐1 cm‐1 can be measured with reasonable accuracy in long‐term experiments and transient changes of 10–8 ohm‐1 cm‐1 can be detected using variations in the capacitive component as index. By means of a graphical method an analysis is made of an equivalent circuit simulating the electrical properties of an electrolyte. It is shown that a series circuit of a varying resistance and a fixed‐value capacitance approximates several of the characteristics of a solution.